Nitrate contamination in groundwater at farmlands in Nsawam, Ghana: The role of fractures from azimuthal resistivity surveys

The role of fractures in nitrate contamination of fractured groundwater aquifers at farmlands within the Nsawam district (Ghana) is investigated using azimuthal square-array resistivity measurements. The study covers seven farming communities where azimuthal resistivity surveys (ARS) were conducted at sites with exposed rocks (outcrops) to measure real and imaginary parts of the electrical conductivity of the rocks, which are then used to provide estimates of anisotropic coefficients and fracture porosities. At each outcrop, fracture orientations, apertures and lengths were measured and used directly to obtain the porosities and specific surface areas of the fractures. Linear regression models were then developed between the specific surface area of the fractures and the electrical parameters. At selected well locations, azimuthal resistivity surveys were performed and the developed regression models were used to estimate porosities and specific surface areas of the subsurface fractures. Groundwater samples were collected from the selected wells to determine the concentration levels of nitrates. The estimated porosities and specific surface areas of the subsurface fractures correlate with concentration levels of nitrate in the groundwater. The investigations suggest that nitrate concentration levels in groundwater in the study area tend to be higher in wells where the specific surface area of the fracture system are lower. Also, wells with lower fracture porosities tend to have lower concentration levels of nitrates. The establishment of linkages between subsurface fracture parameters obtained non-invasively from surface geophysics and nitrate concentration levels is a useful result that can be utilized in environmental impact assessment in complex fractured terrain, such as that of the study area.